Some random people cannot submit forms

Something really weird is happening
Basic background: I have a basic site where people submit forms from time to time during “campaign times”. So like once or twice a month I get around 500 submissions, the gap between “campaigns” also vary and can be as long as 2 months apart.

The problem started happening during our last 2 campaigns (so around mid-march). I started getting reports that some people were not able to submit the form, even if they tried in multiple PCs/mobiles. Even though I never had this problem before (and I always used wpforms), I initially disregarded this as people with old outdated machines and people with little “internet knowledge”. Then we did a “big” campaing that had a little more than 1k submissions and the reports skyrocketed. I got dozens of reports of people not being able to submit a form. People sent me “video proof” of them clicking on the SUBMIT button and it would blink gray and immediatly go back to normal state as if waiting to be clicked again and the form wasn’t submitted, also got a screenshot of a form with “The Form Was Unable To Submit. Please Contact The Site Administrator.” (translated) in red letters right before the form, although this was not present in most of the other videos/screenshots. People were reporting this problem using phones and desktops.

I NEVER had any report of this type before, people not being able to submit forms, and I’ve been using wpforms for more than a year without any problem of this type.
I also was NEVER able to reproduce this error by myself. I tried using multiple phones and desktops, different browsers, never had any problem to submit the form.
I updated EVERYTHING in wordpress after the reports started arriving and it didn’t fix the issue. Reports still coming.
Recaptcha (or any captcha) is not enabled.
AJAX submission is disabled (always was).
Anti-spam protection is currently disabled and didn’t solve the issue, it was originally enabled (default?).
Site doesn’t have ANY Cache plugin.
Tried disabling Wordfence and it didn’t help.
There is nothing in error_log or ERRORS in cPanel about this or wordpress/wpforms.
How can I even start to debug this? I have no idea how to tackle this and I don’t even have errors to report, the site indicates no errors of any kind. Is there some other debugging method inside wpforms so I can monitor this? Is this a known issue that is happening with current version of wpforms that didn’t happen before?